The scale of the problem the authors address is staggering. Since the 1950s, humanity has produced billions of tonnes of plastic, the vast majority of which has never been recycled and instead accumulates in landfills, rivers, and oceans. Microplastics and nanoplastics are now documented in marine and terrestrial ecosystems, in the guts of animals ranging from whales to earthworms, and even in the human digestive tract, where recent studies have identified PET-degrading enzyme candidates in the gut microbiome. Beyond the physical contamination, the plastic lifecycle contributes substantially to greenhouse gas emissions, linking the pollution crisis directly to climate change. Conventional mechanical recycling degrades polymer quality with each cycle, and chemical recycling often demands intense heat and petrochemical inputs, which is precisely why biological routes have attracted such intense scientific attention.

The turning point in plastic biodegradation came in 2016, when Japanese researchers described Ideonella sakaiensis, a bacterium capable of degrading and assimilating polyethylene terephthalate, the ubiquitous plastic known as PET. The bacterium deploys two enzymes in sequence: PETase, which hydrolyzes the polymer into mono-(2-hydroxyethyl) terephthalate (MHET) and related intermediates, and MHETase, which cleaves those intermediates into the monomers terephthalic acid and ethylene glycol. That discovery ignited a global hunt for plastic-degrading enzymes, and subsequent surveys have found PETase-like enzymes with functional motifs distributed across the world’s oceans, in glaciers, in deep-sea sediments, and among uncultured marine microorganisms. Machine learning and multi-omics databases such as PAZy and PlasticEnz are now accelerating the discovery of previously hidden candidate enzymes, dramatically expanding the known catalytic repertoire.

Wild-type enzymes, however, are far too slow for industrial use, and this is where enzyme engineering has transformed the field. Directed evolution campaigns have produced thermostable PET depolymerases that operate at temperatures approaching the glass transition of PET, where the polymer chains become more mobile and accessible. Computational redesign strategies, including the GRAPE approach and machine learning-aided hydrolase engineering published in Nature, have yielded variants capable of nearly complete PET depolymerization at industrially relevant high-solids loading. Researchers have also engineered the expression side of the equation, optimizing signal peptides such as pelB and SPamy to boost secretion of PETase from Escherichia coli and Bacillus subtilis, and displaying engineered PETase and MHETase on bacterial cell surfaces to create efficient dual-enzyme cascade systems. Multi-enzyme cascade strategies and self-assembled complexes now push degradation rates further, and PET enzymatic recycling has reached pilot scale, marking a genuine milestone on the road to commercialization.

PET is not the only target. Polyurethanes, found in foams, adhesives, and coatings, are attacked by enzymes including urethanases, esterases, and lipases, with recent multi-omics studies revealing how marine fungi degrade these polymers. Polyethylene, the most produced plastic and chemically the most recalcitrant, is far harder: its carbon-carbon backbone resists hydrolysis, and degradation typically requires oxidative enzymes such as alkane hydroxylases and, more recently, small laccases, including one discovered in Acinetobacter dijkshoorniae that shows potential for polyethylene depolymerization and bio-upcycling. Polystyrene degradation has been documented in bacteria such as Exiguobacterium, with evidence for atypical oxygenase-mediated pathways, and multi-omic analyses of plastic-associated microbial communities continue to reveal novel enzymes, dubbed plastizymes, with unexplored biotransformation potential. Pretreatment technologies, including UV irradiation, plasma, and thermal conditioning, are increasingly recognized as key enablers that make polymers vulnerable to enzymatic attack.

Degrading plastic is only half of the circular equation; the other half is turning the resulting monomers and renewable carbon into new materials. Here the review highlights polyhydroxyalkanoates (PHAs), a family of microbially synthesized polyesters that behave like conventional plastics yet biodegrade readily in soil and marine environments, and that already show promise in biomedical applications such as wound treatment and food packaging. Metabolic engineering has been decisive in making PHA production economically viable. Researchers have knocked out competing fermentation pathways in E. coli to channel carbon into PHB, disrupted PHA depolymerase genes in Rhodobacter sphaeroides to prevent the polymer from being consumed, and overexpressed biosynthetic gene clusters to raise yields. In halophilic chassis such as Halomonas bluephagenesis, scientists have even manipulated the size of intracellular PHA granules, which influences polymer processing properties, while fine-tuning monomer composition to tune material performance from rigid to elastic.

Perhaps the most exciting frontier is the direct coupling of the two halves of the cycle: converting plastic waste itself into new bioplastic. Pseudomonas putida KT2440 has emerged as the leading chassis for this kind of plastic upcycling, engineered to metabolize both terephthalic acid and ethylene glycol, the two monomers released from PET, and to funnel them into value-added products including PHAs and beta-ketoadipic acid. Tandem chemical deconstruction followed by biological upcycling has been demonstrated for PET, and engineered strains have been built for concurrent PET monomer metabolism and hydrolase expression, effectively creating synthetic PETrophy. Even more striking, I. sakaiensis itself has been shown to directly convert PET into PHA fermentatively, and co-cultivation strategies pair a PET-degrading strain with a PHB-producing strain to achieve one-step degradation and biosynthesis. Similar logic applies to polyurethane monomers, which defined microbial mixed cultures can convert into bioplastic, and to mixed microbial communities that produce PHA from terephthalic acid.

Synthetic consortium design adds another layer of sophistication. Engineered microbial division of labor, in which different strains specialize in depolymerization, monomer transport, and product synthesis, has been shown to improve plastic upcycling beyond what single strains achieve. Artificial consortia have also been used to convert inexpensive feedstocks such as inulin and glucose-xylose mixtures into medium-chain-length PHAs, and photoheterotrophic communities can co-produce biohydrogen alongside PHA. These systems mirror the modular logic of industrial chemistry but operate at ambient temperature and pressure in water, offering a potentially far lower energy footprint than thermochemical recycling routes such as pyrolysis, which nonetheless remain part of the broader circular economy toolkit.

The authors are candid about the bottlenecks that remain. Enzyme stability and catalytic efficiency still fall short of the throughput needed for bulk waste streams, particularly for polyolefins like polyethylene and polypropylene, where degradation rates remain orders of magnitude below those for PET. Process economics, downstream separation, feedstock logistics, and the cost of engineered strains all weigh against bioplastic prices that must compete with commodity petroplastics. The review also notes that some researchers caution microbial enzymes will offer limited solutions to the global pollution crisis on their own, underscoring that biology must be integrated with policy, sorting infrastructure, and system-level design rather than seen as a silver bullet. The authors call for interdisciplinary collaboration spanning strain engineering, protein design, bioprocess innovation, and environmental systems analysis to overcome these barriers.

What emerges from the BRIN team’s synthesis is a coherent vision: microorganisms as the connective tissue of a circular plastics economy, digesting yesterday’s bottles and packaging and secreting tomorrow’s materials. With PET enzymatic recycling already demonstrated at pilot scale, PHA strains genetically fine-tuned for yield and composition, and upcycling routes from waste plastic to new bioplastic proven in the laboratory, the pieces of the puzzle are increasingly in place. The remaining challenge is less one of discovery than of integration and scale, of turning elegant metabolic pathways into robust industrial processes. If that integration succeeds, the microbes that evolved in a world saturated with plastic may become the very tool humanity uses to unsaturate it.

Subject of Research: Microbial metabolic engineering for plastic degradation and bioplastic synthesis in a circular plastics economy

Article Title: The role of microbial metabolic engineering for circular plastics economy

Article References: The role of microbial metabolic engineering for circular plastics economy. (n.d.). https://doi.org/10.1007/s13205-026-05050-0

Image Credits: AI Generated

DOI: 10.1007/s13205-026-05050-0

Keywords: metabolic engineering, plastic degradation, PETase, polyhydroxyalkanoates, bioplastics, circular economy, Ideonella sakaiensis, enzyme engineering, Pseudomonas putida, biodegradation, plastic recycling, microbial consortia